Edit the material In Model, then drag the thumbnail from the In Model pane to the appropriate library. You have to open the secondary pane to drag from the In Model to the other library.
It makes more sense to create your own collection of materials, though. When you upgrade to the next version of SketchUp, you can migrate the custom textures/materials. And if you ever have to reinstall SketchUp, the modified textures in the OEM folders will get overwritten and you’ll lose your work.
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